1. Field of the Invention
The present invention relates to a system for monitoring a workpiece during laser shock processing, and in particular, a system for monitoring the pressure pulse applied to a workpiece during laser shock processing.
2. Description of the Related Art
Laser shock processing involves a pulse of coherent radiation to a piece of solid material (workpiece) to produce shockwaves therein. The produced shockwave cold works the solid material to impart compressive residual stresses within the solid material. These compressive residual stresses improve the fatigue properties of the solid material.
Current laser shock processing utilizes two overlays: a transparent overlay (usually water), and an opaque layer (usually an oil based paint or black plastic tape). During processing, a laser beam is directed to pass through the transparent overlay and is absorbed by the opaque layer, causing a rapid vaporization of the opaque layer (plasma creation) and generation of a high-amplitude shockwave. The shockwave cold works the surface of the part and creates deep compressive residual stresses which provide an increase in fatigue properties of the workpiece. A workpiece is typically processed by employing a matrix of overlapping spots that cover the fatigue-critical zone of the part.
Currently, there is no known real-time method for measuring the shock pressure applied to a workpiece during laser shock peening. While commercial pressure gauges, such as special quartz gauges or PVDF gauges are available to make pressure measurements, these gauges must be used offline (not in real time on a workpiece). Furthermore, these gauges are single-use devices.
A quartz gauge is based on the piezoelectric behavior of quartz crystals. When a pressure is applied to one surface of a quartz crystal, an electric current proportional to the stress difference between this surface and the opposite surface is produced between electrodes attached to these surfaces. The current then passes through a resistor and the voltage measured across the resistor is proportional to the difference in the stress between the opposite surfaces. In the xe2x80x9cthick gaugexe2x80x9d mode, most or all of the shockwave passes into the thickness of the gauge before it reaches the opposite surface of the crystal. This enables one to measure the entire shockwave profile directly.
One problem with current laser shock processing systems is that there is no real-time method or apparatus for measuring shock pressure or plasma characteristics during laser shock peening or correlating them to the imparted deep compressive residual stresses in a workpiece. Previous methods of measuring a pressure pulse applied to a workpiece included use of a quartz gauge. The disadvantage of using a quartz gauge is that a quartz gauge is a single use instrument. In addition, the use of a quartz gauge does not permit real-time measuring of a pressure pulse while processing a workpiece. The use of a quartz gauge is limited to measuring the pressure pulse applied to a workpiece either before or after laser shock processing (i.e., not real time).
Another problem in the art is that there is no method for correlating plasma characteristics to a pressure pulse applied to a workpiece.
Another problem in the art is that there is no known method or apparatus for real-time determination of imparted compressive residual stresses in a workpiece during laser shock peening. Currently, the method of evaluating an imparted residual stress profile is to measure the residual stresses using x-ray diffraction techniques. In order to use x-ray diffraction, a workpiece is normally removed from the laser shock processing station and placed in an x-ray machine, wherein an x-ray beam is directed to the workpiece surface to measure the residual stresses at that surface. In order to get an in-depth profile, a sequence of thin layers is removed from the surface by electropolishing, then the surface residual stresses are measured between each electropolishing step. If only the residual stress at the original surface of the workpiece is measured, the measurements also includes the unknown effects of previous surface finishing processes. These usually vary from part to part and could be differentiated from the stresses imparted by laser shock peening. The technique of x-ray diffraction for in-depth profiles is a destructive method for evaluating compressive residual stresses imparted in a workpiece by laser shock peening. The use of x-ray diffraction is limited to post-laser shock peening analysis. Therefore, x-ray diffraction cannot be used as a real time method for determining imparted compressive residual stresses.
The present invention is a method and apparatus for monitoring laser shock processing of a workpiece. The method and apparatus includes detecting spectral and acoustic energy emitted from a workpiece or the energy absorbing layer applied to the workpiece. The acoustic and spectral emissions may be correlated to the pressure pulse applied to a workpiece, the residual stress profile produced in the workpiece, and the fatigue life of the workpiece.
The invention, in one form thereof, is an apparatus for monitoring laser shock processing of a workpiece. The apparatus includes a material applicator for applying an energy absorbing material to the workpiece. A transparent overlay applicator applies a transparent overlay onto the workpiece over the energy absorbing layer. A laser is operatively associated with the energy absorbing layer and there is at least one radiometer. In one particular further embodiment, the energy absorbing layer may contain a dopant.
The invention, in another form thereof, is a method for real time monitoring laser shock processing of a workpiece. The method includes applying an opaque overlay to the workpiece. A beam of coherent energy is directed to the workpiece to vaporize a portion of the opaque overlay and to create a plasma which emits energy therefrom. A portion of the energy emitted from the plasma is monitored. In a further embodiment, spectral emissions are detected from the emitted energy. In an alternate embodiment, acoustic emissions are detected within the emitted energy.
The invention, in yet another form thereof, is a method for real time monitoring the laser shock peening of a workpiece. The method includes applying a transparent overlay to a workpiece and directing a beam of coherent energy to the workpiece through the transparent overlay and to create a plasma which emits energy therefrom. A portion of the energy emitted from the plasma is monitored. In a further embodiment, spectral emissions are detected from the plasma emitted energy. In an alternate embodiment, acoustic emissions are detected from the emitted energy. In alternate further embodiments, a feature of the spectral emissions or acoustic emissions are correlated to the shock pressure applied to the workpiece.
The invention, in another form thereof, is a method for real time monitoring the laser shock peening of a workpiece. The method includes applying an opaque overlay to a workpiece and directing a beam of coherent energy to a workpiece to vaporize a portion of the opaque overlay and create a plasma thereon. The temperature of the plasma is monitored. In a further embodiment, the plasma temperature is correlated to the residual stress profile left in the workpiece.
The invention, in yet another form thereof, is a method for real time monitoring the laser shock peening of a workpiece. The method includes applying a transparent overlay to a workpiece. A beam of coherent energy is directed to a workpiece through the transparent overlay and creates a plasma thereon. The temperature of the plasma is monitored. In further alternate embodiments, the plasma temperature is correlated to the shock pressure of the workpiece, the residual stress profile left in the workpiece, and the fatigue life of the workpiece.
One advantage of the present invention is the ability to correlate characteristics and emissions from a workpiece and overlay during laser shock peening to a shock pressure, compressive residual stress profile, or fatigue life of a workpiece. Characteristics or features, such as acoustic emissions and spectral emissions are correlated to a shock pressure, compressive residual stress, or fatigue life of a workpiece. As a result, monitoring such characteristics allows one to predict the shock pressure and compressive residual stresses or fatigue life of a workpiece in real time.
Another advantage of the present invention is a non-destructive technique to predict the compressive residual stresses imparted in a workpiece during laser shock processing. Through the correlation of acoustic and spectral emissions from a workpiece or overlay to a shock pressure and residual stress profiles imparted to the workpiece, one can determine the effectiveness or success of the laser processing of the workpiece. Therefore, it is no longer necessary to use a destructive technique, such as x-ray diffraction, to evaluate the imparted compressive residual stresses during laser shock processing.
Another advantage of the present invention is the ability to do real time quality control of a workpiece subjected to laser shock processing. Since measured characteristics of acoustic and spectral emissions may be correlated to shock pressure and compressive residual stresses imparted in a workpiece, one can perform quality control in real time of a laser shock processed workpiece. By measuring acoustic or spectral emissions, one can determine the imparted compressive residual stress of a workpiece. If the correlated compressive residual stress is not within a predetermined or desired range, additional laser shock processing may be done to ensure the desired compressive residual stress in the workpiece is achieved.
Prior to this invention, there was no known real time technique available for determining the compressive residual stresses imparted in a specific workpiece to provide quality control of that specific workpiece. Prior to this invention, a technique of random or regular testing was done on a workpiece subsequent to laser shock peening a lot of workpieces. The randomly selected workpiece was subjected to the destructive technique of x-ray diffraction in which the imparted compressive residual stress was measured in the randomly selected workpiece. The results of the randomly selected workpiece""s residual stress profile was then extrapolated to non-tested workpieces processed under the same condition, to estimate the non-tested workpieces"" imparted compressive residual stress profile.